Field
Embodiments relate to the field of circuit protection devices, and more particularly to a semiconductor devices for protection against transient overvoltage.
Discussion of Related Art
Semiconductor devices are widely used to provide protection against transient conditions, such as transient overvoltage events by taking advantage of the properties of P/N junctions. In a P/N junction an interface is formed between a region of the semiconductor device having a first conductivity type (P or N) and a second region having a second conductivity type opposite the first conductivity type (N or P). To form some conventional transient protection devices, a semiconductor substrate having a conductivity of a first type is exposed to implantation, diffusion, or deposition of species of a second type, including epitaxial growth of a layer having species of the second type. After the species of the second type is provided, annealing may be performed to diffuse and activate the species of second conductivity type. In this manner a Zener diode or avalanche breakdown diode may be formed for limiting voltage to levels of several volts to several hundred volts.
One disadvantage of using a single PN junction diode is that series resistance within the diode increases the voltage drop across the diode during high current transients. This increases the voltage window between the non-conducting voltage and high current clamping voltage and therefore limits the maximum operating voltage of the protected circuit. In applications such as switching inverters, it may be useful to operate the circuit at the highest possible switching voltage for highest efficiency. A wide protection voltage window may compromise the efficiency of the switching inverter.
In some instances an avalanche voltage temperature coefficient is in the range of 0.1% per degree Celsius for a silicon PN diode. Internal heating of the PN junction occurs during a current transient and this heating causes and increase in the avalanche voltage. A protection voltage window may accordingly be increased to include the effects of temperature rise during operation.
In some applications, it may be useful to protect AC voltages. In these circumstances, a pair of PN diodes may be connected back-to-back in series to provide protection for two opposite voltage polarities. In this situation, the sum of the series resistance from the forward conducting diode and the avalanching diode again increases the protection voltage window.
In an attempt to reduce the series resistance of the protection diode, the device may be fabricated on an epitaxial layer of silicon grown on a low resistivity substrate. The PN junction may be formed in the epitaxial layer by similar methods described for the non-epitaxial diode. Notably, the problem of the finite temperature coefficient of avalanche voltage that expresses the change in avalanche voltage with change in temperature remains.
In other work an NPN device structure has been fabricated by forming an n-region on opposite sides of a p-doped semiconductor die. Known methods for forming overvoltage protection devices in semiconductors are capable of generating single devices having breakdown voltage values in the range 15-30V, with possibilities to extend breakdown voltage to 40 V. This structure also behaves like a transistor in the avalanche mode. Electrons injected into the P-type base region from the N-type emitter diffuse across the P-type base and reach the avalanche region at the collector. These additional charge carriers reduce the avalanche voltage of the collector junction to give the known foldback BVceo (breakdown voltage, collector-emitter, base open) characteristic of a transistor. This effect is used to advantage in the 15-30V avalanche voltage range to compensate for the voltage increase due to series resistance. The gain of a transistor depends on the doping of the base region, and the gain increases as the doping is reduced. If the transistor gain is too high then, BVceo can reduce to lower than the stand-off voltage of the protection diode. In practice, this consideration means the maximum voltage of the protection diode is limited to about 30V. In view of the above, it may be useful to provide overvoltage protection for higher voltage levels. It is with respect to these and other issues the present improvements may be desirable.